Morphological Evolution of Galaxies

Abstract

We simulate the growth of large-scale structure for three different cosmological models, an Einstein-de Sitter model (density parameter Ω0 = 1), an open model (Ω0 = 0.2), and a flat model with nonzero cosmological constant (Ω0 = 0.2, cosmological constant λ0 = 0.8), using a cosmological N-body code (particle-particle/particle-mesh) with 643 dark matter particles in a comoving cubic volume of present comoving size 128 Mpc. The calculations start at z = 24 and end at z = 0. We use the results of these simulations to generate distributions of galaxies at the present (z = 0), as follows: Using a Monte Carlo method based on the present distribution of dark matter, we located ~40,000 galaxies in the computational volume. We then ascribe to each galaxy a morphological type based on the local number density of galaxies in order to reproduce the observed morphology-density relation. The resulting galaxy distributions are similar to the observed ones, with most ellipticals concentrated in the densest regions, and most spirals concentrated in low-density regions. By tying each galaxy to its nearest dark matter particle, we can trace the trajectory of that galaxy back in time by simply looking at the location of that dark matter particle at earlier time slices provided by the N-body code. This enables us to reconstruct the distribution of galaxies at high redshift and the trajectory of each galaxy from its formation epoch to the present. We use these galaxy distributions to investigate the problem of morphological evolution. Our goal is to determine whether the morphological type of galaxies is determined primarily by the initial conditions in which these galaxies form or by evolutionary processes (such as mergers or tidal stripping) occurring after the galaxies have formed and eventually altering their morphology, or a combination of both effects. Our main technique consists of comparing the environments in which galaxies are at the epoch of galaxy formation (taken to be at redshift z = 3) with the environment in which the same galaxies are at the present. Making the null hypothesis that the morphological types of galaxies do not evolve, we compare the galaxies that form in low-density environments but end up later in high-density environments to the ones that also form in low-density environments but remain in low-density environments. The first group contains a larger proportion of elliptical and S0 galaxies than the second group. We assume that the initial galaxy formation process cannot distinguish a low-density environment that will always remain low density from one that will eventually become high density. Therefore, these results are absurd and force us to discard the null hypothesis that morphological evolution does not occur. Our study suggests that ~75% of the elliptical and S0 galaxies observed at present formed as such, while the remaining ~25% of these galaxies formed as spiral galaxies and underwent morphological evolution for all three cosmological models considered (the percentages might be smaller for elliptical than for S0 galaxies). These numbers assume a morphological evolution process that converts one spiral galaxy into either a S0 or an elliptical galaxy. If the morphological evolution process involves mergers of spiral galaxies, these numbers be would closer to 85% and 15%, respectively. We conclude that most galaxies did not undergo morphological evolution, but a nonnegligible fraction did.